1. Field of the Invention
This invention relates generally to the field of measurement-while-drilling tools. More particularly, this invention relates to methods for processing electromagnetic signals from a measurement-while-drilling tool to model and visualize layered subterranean earth formations surrounding the tool.
2. State of the Art
Wellbores drilled through earth formations to extract fluids such as petroleum are frequently drilled along a substantially horizontal trajectory in a reservoir in order to increase the drainage area in the reservoir. Because such reservoirs are frequently located in layered subterranean earth formations, the position of the substantially horizontal wellbore relative to the boundaries of the layers in the earth formation often has a material effect on the productivity of such wellbores.
Geosteering solutions have been developed that utilize a measurement-while-drilling tool to provide for real-time prediction and visualization of the layer structure of the subterranean earth formation surrounding the tool. Such real-time visualization allows operators to control the direction of the well bore drilling operations in order to place (i.e., land) the wellbore in a particular section of a reservoir to minimize gas or water breakthrough and maximize economic production therefrom.
Electromagnetic (EM) induction and propagation-style logging tools are well suited for these geosteering applications because of their relatively large lateral depth of investigation into the surrounding formation. Directional EM measurement-while-drilling tools have recently been proposed; see Seydoux et al., “A Deep-Resistivity Logging-While-Drilling Device for Proactive Geosteering,” The Leading Edge, Vol. 23, no. 6, pp 581-586, 2004; Li et. al., “New Directional Electromagnetic Tool For Proactive Geosteering And Accurate Formation Evaluation While Drilling,” 46th SPWLA Annual Symposium, Jun. 26-29, 2005; and Yang et. al., “Bed-Boundary Effect Removal to Aid Formation Resistivity Interpretation from LWD Propagation Measurements at All Dip Angles”, SPWLA 46th Annual Logging Symposium, New Orleans, Jun. 26-29, 2005, all incorporated by reference herein in their entireties. Such directional EM measurement-while-drilling tools enable distinguishing the resistive properties of the formation above and below the tool location while drilling substantially horizontal and deviated wells. These measurement-while-drilling tools routinely carry both relatively short coil spacings sensitive to the EM properties of the formation layers near the logging tool location as well as longer coil spacings that are also sensitive to the resistive properties of formation layers farther away from the tool location.
When drilling through some earth formations, the directional EM measurement-while-drilling tools are sensitive to formation properties quite distant from the instrument location, while in other formations, the tools are only sensitive to more local formation properties near the instrument.
Current geosteering solutions provide for modeling and visualization of the formation properties near the tool locations (see commonly owned, U.S. Pat. No. 6,594,584, entitled “Method for Calculating a Distance Between a Well Logging Instrument and a Formation Boundary by Inversion Processing Measurements from the Logging Instrument”, incorporated herein be reference in its entirety). Current geosteering solutions, however, fail to provide an effective mechanism for modeling and visualizing formation properties far away from the tool locations. Thus, in some earth formations, the formation properties and boundary locations far away from the tool location are only partially determined from the measurements, and it is difficult to locate (e.g., land) the wellbore in a desired section of the reservoir that maximizes economic production without quantifying and effectively displaying this partial information.
Current methods for displaying locations where inverted resistivity is unreliable through the use of color saturation has been published previously by Oldenburg, et al., “Estimating depth of investigation in DC resistivity and IP Surveys,” Geophysics Soc. of Expl. Geophys., Vol. 64, pp 403-416, 1999, incorporated by reference herein in its entirety. These methods (a) do not use uncertainty but a “depth of investigation” (DOI) index to indicate where resistivity is unconstrained by the data and (b) are applied to surface resistivity measurements. Accounting for uncertainty displaying the partial information available from the measurements concerning formation properties far from the tool locations.
Moreover, in some instances, the earth formation has been surveyed by other means (e.g., a 3D seismic survey) prior to drilling. In such instances, it would be beneficial to enable a comparison of the formation properties visualized by the geosteering solution to the formation properties of the prior survey to allow for more informed decision-making regarding the directional control over the drilling operations. Because the current geosteering solutions fail to provide an effective mechanism for modeling and visualizing formation properties far away from the tool locations, it is difficult to confirm with certainty correspondence between the current drilling location and the prior survey. This makes it difficult to effectively integrate the information of the prior survey with the real-time information generated by geosteering and thus can limit the drilling control decision-making process.